It is common practice in the valve art to line the bodies of valves with a corrosion-resistant material such as the polytetrafluoroethylene plastic known as Teflon (trademark of DuPont Company). If this is done properly, the entire interior of the valve is prevented from contacting corrosive slurries or fluids, the flow of which can then be controlled by the valve without its untimely demise. However, prior art Teflon-lined valves have not been as successful as might be desired because of the complexity of forming adequate seals in the valve, to prevent leakage both in the line and also around the stem by which the valve plug is rotated from its open to its closed position. Undesirably complex expedients have been tried, such as seals running around the orifices in the plug which mate with the axial flow tubes formed in the valve body. See for example commonly-assigned British Pat. No. 2,017,265. However the seal made according to that patent is very difficult to manufacture. Complex stem seal arrangements have also been tried but, as noted, none have been entirely successful.
In particular, all prior art plug valves of which the present inventor is aware require a gland for the stem seal, which is adjusted to compensate for wear over time. See U.S. Pat. Nos. 3,916,943 to Hester el al and 3,522,820 to Watts et al, both of which acknowledge the problem of stem seal, or "gland" wear. The problem is particularly difficult in the case of lined valves.
As used in this specification and the appended claims, a "plug valve" is to be understood to be a valve in which a tapered plug, having an included angle on the order of 8.degree., fits tightly within a similarly tapered bore. The plug rotates 90.degree. about the axis of the taper for opening and closing. The provision of the taper means that the valve plug need not have a rigidly fixed axial position with respect to the valve body, so that it can be biased into firm engagement with the bore to effect a good seal between the taper of the plug and that of the bore. This arrangement also permits the valve body and plug to move somewhat with respect to one another, e.g., upon temperature-caused expansion and contraction, without destruction of the seal. This is in clear distinction to other valve types such as butterfly valves, in which a generally circular disk is pivoted about an axis perpendicular to the axis of a generally cylindrical passageway. In order that a seal can be formed, the edge of the circular disk is forced into deforming engagement with the wall of the passage which is typically provided with a resilient wall structure, e.g., a rubber member behind the liner in the case where the butterfly valve is lined with Teflon or the like. Unlike a plug valve, axial up and down motion of the butterfly valve member is not permitted and steps must be taken to insure that thermal expansion and contraction does not destroy the seal of such valves. As in the case of the plug valve, the butterfly valve requires at least one stem seal as well.
One particular type of butterfly valve which is relevant to the present invention is one in which the stem seal comprises a ring fitting into mating annular grooves formed around the stem. If a bottom stem pivot is also used, the same arrangement of a ring fitting within mating grooves can be used. The ring can be urged outwardly by resilient O-rings also located in the mating annular grooves acting on surfaces formed in the annular ring. This structure has been used in butterfly valves made and sold by the assignee of the present invention, as shown in Bulletin 792-B published by the assignee, Garlock, Inc. However, as noted, butterfly valve and plug valve design have generally been treated differently in the art due to the differing ways by which the body seal is made. In particular, a plug valve construction must permit some axial motion of the plug with respect to the bore, if it is to be readily manufacturable, i.e., so that end-to-end manufacturing tolerances can be made reasonable, and also so that expansion and contraction with temperature changes do not destroy the seal. On the other hand, butterfly valves have extremely critical end-to-end tolerances, but the body seal is formed by a deformation of the body of the valve upon engagement thereof with the butterfly disk. Temperature change does not significantly affect the seal. In the tapered plug valve case, the planar contact between the tapered plug and the mating taper of the bore affects the body seal, i.e., the seal of the plug to the body of the valve.
Those skilled in the art will note, as discussed above, that no plug valve design has been entirely adequate to satisfy the needs of the art, particularly with respect to lined plug valves.